AZW3 is like epub. It's KF8 format so its settings wont be overwritten like a mobi file is, Sideloaded because KF8 isn't supported through the personal documents service.

Mantano works fine on the HD. The newer versions is just buggy. I paid $5 for it when it was on sale. It must be sideloaded though. Amazon wont let you install it from the app store. It's a whole competition thing. You can buy it from SlideME or use another device to get it from the app store using like an Android cell phone.

Mantano is one of the best epub eReader apps out there. It has collections and everything you could want in a epub app. It's totally customizable.

Thanks Blossom. I see the latest version is at Amazon App store, but it says it is not for Kindle Fire HD. If you buy an app from Amazon and it says it is not compatible does that mean no returns?

If this works for changing fonts couldn't you have used this for a new KFHD? Just wondered. You are so helpful with the Fire I will be sorry to see you go to another toy.